1. Field of the Invention
The present invention relates to a network for transferring data packets, with multiple user stations and at least one central station which connects the network externally, the data packets being transferred over a set of channels with the frequency-hopping method and the channels being in this context selected for data transfer in accordance with at least one frequency-hopping pattern in temporal succession; as well as a method for operating a network of this kind.
2. Related Prior Art
A network of this kind and a method for operating it are known from DE 44 07 544 A1.
The known network and the known method are used to transfer data packets in an auxiliary network which transmits via frequency channels that are at least in part already used in an existing wireless or radio network for digital data and/or voice transfer. With this method, in a first step a frequency channel not currently occupied by the wireless network is identified, whereupon in a second step a data packet is transferred over the identified frequency channel. These steps are repeated, using frequency-hopping technology, until all the data packets of a transmission have been transferred.
With this method, the determination of a frequency channel not currently occupied by the wireless network takes place in such a way that one of the multiple frequency channels is first selected, whereupon this selected channel is then listened in on to check whether the wireless network is currently transmitting on that frequency channel. If no signal is received during this listening-in process, it is assumed that this channel can be used by the auxiliary network.
If it is found, however, that the selected channel is currently being used by a primary user, the time slot elapses unused, i.e. no data packet is transferred over the selected channel so as not to disrupt the primary user.
With the known method, by using frequency-hopping technology it is possible to utilize existing channels better without disturbing the respective wireless network.
As is common in frequency-hopping technology, the individual data packets are transferred in temporally staggered fashion over various frequency channels, the load being evenly distributed among the available channels that are not currently being used by the wireless network.
With regard to further details of the known method, reference is made to DE 44 07 544 mentioned above.
Initial tests of the known method have revealed that it operates properly without impairing the wireless network. Operation of an auxiliary network on the D1 or D2 network therefore presents no problems.
The known network is a decentralized network in which the central station on the one hand specifies the internal system time and on the other hand provides the external connection for the individual user stations. The user stations can communicate both with one another and with the central station.
A separate frequency-hopping pattern is assigned to each user station, so that each user station is reachable at any time. The channel over which this is possible is determined from the system time and the address of the particular user station. To prevent collisions from occurring in the network, the frequency-hopping patterns of the individual user stations are orthogonal to one another, i.e. at any given point in time, each of the available channels is used only once within the network.
If the frequency range allows 80 different channels, for example, 79 user stations and one central station can therefore be provided in the network. A network of this kind can be connected via the central station to a further network also with 79 user stations and one central station.
Since multiple transmissions can be carried out simultaneously within a network, data throughput through the network is very high, even though it is simply overlaid on the wireless network of primary users.
Although the network of user stations so far described operates very reliably with high data throughput, a number of disadvantages leading to a perceptible reduction in data throughput have nevertheless been noted during operation.
For example, it often happens that two adjacent user stations which are associated with two adjacent networks accidentally receive data over the same channel because their two frequency-hopping patterns have the same channel in the current time slot. If these two user stations are located sufficiently close to one another, they then interfere with each other so that both data packets are lost. The reason is that the user stations cannot unequivocally assign the data packets, so that they also cannot confirm receipt of the data packet to the transmitting station. The lost data packet must then be re-transferred in the next time slot in each network.
These types of interference occur particularly at the edges of a network, where it touches or in fact overlaps adjacent networks.
A further problem with the known network crops up if one of two adjacent user stations is currently transmitting while the other has switched over to receiving. Although these two user stations are receiving or transmitting data over different channels, the transmitting user station still prevents reception by the other user station, since its strong transmitted signal can lead to saturation of the input amplifier of the other user station. When saturation of this kind occurs, the receiving user station is no longer capable of distinguishing between the data packet intended for it and the data packet, not intended for it, being sent out by the adjacent user station. The data packet intended for the receiving user station is thus lost, so that it must be repeated in the next time slot.
Particularly when the user stations have a high spatial density, the two problems described above cause a large number of the transferred data packets to "get lost," so that the potential data throughput is by no means achieved. The greater the number of user stations in a particular area, the more evident the aforesaid problems become.